Conventionally, a lighting optical apparatus, which is used as a light supply in an image display system used in a liquid crystal projector or the like, usually comprises a light source and a reflecting mirror, which are integrated into one unit. Examples of the light source include halogen lamps, metal halide lamps, xenon lamps, extra-high pressure mercury lamps, and the like.
Recently, because of its good efficiency, high luminance, good balance of red, blue and green in the emitted light, long lifetime, and others, an extra-high pressure mercury lamp having a short electrode spacing, that is, with a short arc, which is dose to a point light source, has been used as a light source for a lighting optical apparatus.
Previously, this type of lighting optical apparatus as shown in FIG. 8, which comprises a high pressure discharge lamp, e.g. an extra-high pressure mercury lamp 17, and a concave reflecting mirror 9 having a paraboloidal or ellipsoidal reflection surface (hereinafter referred to as the reflecting mirror 9) integrated into one unit, has been used.
The light radiated from the extra-high pressure mercury lamp 17 is reflected by the reflecting mirror 9 and then it is radiated forward. If such a lighting optical apparatus is combined with an image display system with a condenser lens or an image forming device such as a liquid crystal panel, the light radiated forward is led into the condenser lens with a determined area, or into the image forming device such as a liquid panel in the image display system.
If the light reflected forward by the reflecting mirror 9 is parallel rays, the condensing efficiency becomes high. Thus, the light source is preferably a point light source. Therefore, an extra-high pressure mercury lamp having a short electrode spacing, i.e. with a short arc, which enables a point light source, may be used.
As an example of a conventional extra-high mercury lamp, FIG. 8 illustrates the extra-high pressure mercury lamp 17 which comprises a luminous vessel 17a containing a pair of electrodes therein, and sealing parts 17b connected to each end of the luminous vessel 17a. An installation body as described below is sealed in each of the sealing parts 17b. The installation body comprises an electrode 18 comprising an electrode rod 18b and a coil 18a connected to the top end of the rod 18b, a metallic foil 5 comprising molybdenum whose one end is connected to the bottom end of the rod 18b, and an external lead wire 6 whose one end is connected to the other end of the metallic foil 5. The installation body is sealed in the sealing part 17b in such a way that the electrode 18 is located in the luminous vessel 17a.
One external lead wire (not shown) is electrically connected to the base 7, and the other external lead wire 6 is connected to a power-supplying wire (not shown).
The luminous vessel 17a is filled with mercury as a light-emitting metal and rare gases, e.g. argon. The extra-high pressure mercury lamp 17 is attached to and integrated with the reflecting mirror 9. The reflecting mirror 9 is made of a material selected from the group consisting of glass, metals and ceramic, and also has a reflecting surface comprising a deposited film of TiO.sub.2 --SiO.sub.2 and the like with excellent reflection property on the inner surface of the concave mirror. A front light-projecting portion of the reflecting mirror 9, i.e. the opening portion, has a diameter of about 50 to 120 mm. The mirror 9 is further provided with a cylindrical support 10 at the back portion thereof. A base 7 of the extra-high pressure mercury lamp 17 is fixed to the cylindrical support 10 with an adhesive 11, e.g. an insulating cement. Thereby, the extra-high pressure mercury lamp 17 is attached to the reflecting mirror 9 in such a way that the axis of the lamp corresponds approximately to the center of the reflecting mirror 9. Furthermore, a lead-in hole (not shown) is formed through the reflecting mirror 9, and above-mentioned power-supplying wire penetrates through the hole and is lead into the back side of the reflecting mirror 9. In the case of power consumption at 80 to 150 W, such a conventional extra-high mercury lamp 17 has an electrode spacing as short as 1.0 to 2.0 mm, and is usually lighted up by a high-frequency alternating current power source at 125 to 400 Hz.
When such a discharge lamp with a short arc and a high luminance is lighted, the temperature at the end of the electrodes becomes very high, so that tungsten used as a material of the electrodes is scattered and adheres to the inner wall of the discharge tube. Thus, blackening of the discharge tube occurs within several tens of hours. In order to inhibit such blackening of the discharge tube, a method of filling a halogen gas in the discharge tube, so as to prevent blackening of the tube by utilizing a reaction called halogen cycle, has been proposed (Japanese Published Unexamined Patent Application (Tokkai) No. HEI 2-148561). The extra-high pressure mercury lamp as proposed in this publication is filled with more than 0.2 mg/mm.sup.3 of mercury, and is also filled with at least one halogen selected from the group consisting of Cl, Br and I in an amount of 10.sup.-6 to 10.sup.-4 .mu.mol/mm.sup.3.
However, in such a lamp, the pressure in the discharge tube during operation exceeds 2.0.times.10.sup.7 Pa (200 bars), so that even a little blackening of the discharge tube can cause deformation of the tube, which may result in bursting of the discharge tube. Furthermore, residual impurity gases remained in the discharge tube, and impurity gases discharged from the electrodes and the quartz glass, which is used as a material of the discharge tube, inhibit the halogen cycle, resulting in shortening the lifetime of the lamp.
Thus, although such a conventional high pressure discharge lamp with a short arc and a high luminance has excellent initial properties, it has a disadvantage with respect to the lifetime of the lamp.